O-rings are super easy to find for most pistols, especially KJ Beretta's.
Go to any home depot, or any hardware store really.
Get o-rings that are used for taps, try and find the mid-small size. I cant remember the exact dimensions but I can find them for my TM 1911 MEU in o-ring kits by MOEN.
KJ Beretta's are tanks. Just be careful with the right side safety.